Electrooptical displays constructed with polymer-coated elements positioned between substrates

ABSTRACT

There is disclosed a liquid crystal display device comprising two substrates facing and spaced from each other, at least one of the substrates being transparent, electrodes positioned to establish an electric field in the space between the two substrates, one or more spacer elements located between the substrates, the spacer elements having been introduced between the substrates during assembly of the device, an electrooptic material filling at least a portion of the space between the two substrates, and a polymeric material filling at least a portion of the space between the substrates, the polymeric material comprising a liquid prepolymeric material that was applied to the spacer elements in liquid form and having been polymerized in situ after the spacer elements were in place between the substrate.

BACKGROUND OF THE INVENTION

This invention relates to liquid crystal and other electronic displays.

Commercially, it is highly desirable for an electronic display to be asthin and light as possible while still maintaining a high degree ofruggedness and imperviousness to forces that are a consequence of shockand drop. In the area of mobile electronics, such as cell phones andpersonal digital assistants (PDAs), size and weight are critical factorsto the commercial success of a product, but currently breakage of thedisplays within these devices remains the primary cause of repairs andproduct returns. In addition, the need for electronic displays that canactually be bent has been acknowledged in several areas: so-called‘electronic paper’ in which fiber paper is replaced with a display wouldbe much more compelling as a product if the electronic display could berolled up or folded like traditional paper; wearable electronics such ascomputers or multifunction watches would be much more comfortable to thewearer if the display were to conform to the user's body; chip cardswhich have strict flexure life-test performance standards would be ableto incorporate flexible displays and still conform to those standards.Replacement of the glass substrates within displays with plastic filmhas been an area of active research within the display community for anumber of years.

Electrophoretic displays achieve images via electrophoretics—the rapidmigration of microparticles in colloidal suspensions. Light scatteringparticles are moved within a dyed colloidal suspension by electrostaticforces. The particles will either move toward the viewer, in which case,the typically white particles are seen by the viewer, or to the surfaceaway from the viewer, in which case, the white particles will be hiddenby the dark dye.

Cholesteric displays are another display technology being attempted onplastic substrates. When sandwiched between conducting electrodes,cholesteric liquid-crystal material can be switched between two stablestates—the so-called focal conic and planar states—in which the liquidcrystal's helical structures have different orientations. In the focalconic state, the helical structures are unaligned and the liquid crystalis transparent. In the planar state, the helical structures' axes areall perpendicular to the display's surface resulting in essentiallymonochromatic transmission by the display.

The Gyricon display being developed by Xerox, is made of microscopicbeads, randomly dispersed and held in place between two plastic sheetsby a flexible elastomeric matrix of oil-filled cavities. The balls havestrongly contrasting hemispheres, black on one side and white on theother. The white side is highly reflective, while the black side absorbslight. Each hemisphere has a unique intrinsic charge, resulting in aforce on the ball when an electric field is applied and the axis of theball isn't aligned with the field. The side of the ball presented fordisplay depends on the polarity of the voltage applied to the electrode.In all three of these cases, while they have some positive features suchas high contrast and compatibility with plastic substrates, they allcurrently high drive voltages, have slow response times, and are notcompatible with commercially available drive electronics.

Liquid crystal displays (LCDs) are attractive because of the low drivevoltages required to switch them, their relatively fast response times,the wide availability of drive electronics, and the significantintellectual and manufacturing investment in the technology. Attemptshave been made to develop LCDs that intermixed the liquid crystal withina polymer matrix in order to make them compatible with plasticsubstrates, one example being polymer dispersed displays (PDLCDs).PDLCDs are fabricated by intermixing the liquid crystal and a prepolymerinto a solution prior to assembling the display. After assembling thedisplay, the polymer is cured, typically by ultraviolet light. Duringthe polymerization the LC separates out from the polymer intomicroscopic droplets. Since the droplets of LC are not in contact withany alignment layer, the orientation of the molecules is random andlight is scattered by the droplets. Applying a voltage to the electrodesof the PDLCD causes the LC molecules to become aligned, resulting in thedisplay becoming transparent. Like the other flexible displays, PDLCDsrequired high drive voltages not generally compatible with existingdrive electronics. Prior art such as U.S. Pat. Nos. 4,688,900,5,321,533, 5,327,271, 5,434,685, 5,504,600, 5,530,566, 5,583,672,5,949,508, 5,333,074, and 5,473,450 all make use of phase separation ofan LC/polymer mixture during polymerization of the polymer using lightas the curing mechanism (photopolymerization).

Methods have been developed to achieve anisotropically dispersedLC/polymer structures which might have drive voltages lower then thoseachieved in PDLCDs. U.S. Pat. No. 5,949,508 describes a method in whicha lamellar structure is achieved whereby the LC and polymer are disposedon opposite substrates; this reduces the drive voltages necessary toswitch the device, but results in a structure where it is only practicalto have the rubbed alignment surface on one of the substrates. Whilethis structure is effective with nematic or electrically controlledbirefringence (ECB) displays, it becomes more difficult to constructdisplays such as twisted nematic (TN) and super twisted nematic (STN)which typically require alignment surfaces on both substrates. U.S. Pat.Nos. 5,473,450 and 5,333,074 describe methods of localizing the polymerduring photopolymerization by exposing only portions of the device tothe light source using masks. Polymer structures of a size on the orderof a pixel (˜0.3 mm) are achievable, but manufacturing may be moredifficult since the photomask must generally be aligned to the electrodestructure within the device and expensive collimated UV light sourcesmust generally be employed. Structures much smaller than 0.3 mm may bedifficult to achieve due to the inherent scattering of the LC/polymermixture. U.S. Pat. No. 5,473,450 teaches the patterning ofphotoinitiator onto the alignment layer, but this method generallyrequires a highly accurate, screened deposition of the chemicalphotoinitiator onto the substrates. Proper alignment of thesilk-screening mask to the clear ITO electrodes may be difficult toachieve, and the introduction of chemicals directly onto the polyimidealignment surface may result in poor alignment of the LC to thealignment surface, poor appearance of the display and lowermanufacturing yields.

Other methods have been developed for providing adhesion between plasticsubstrates involving adhesive elements. In one method, a thermoset(e.g., epoxy) or thermoplastic (e.g., hot melt) adhesive as a solidcoating around the spacer elements (U.S. Pat. Nos. 4,678,284,5,130,831). Heat and pressure are applied after the coated spacers arein place between the substrates. Other methods use solid adhesiveelements separate from the spacer elements (U.S. Pat. Nos. 5,812,232,5,581,384, 6,004,423).

In addition to the breakage problems due to shock and drop, glasssubstrate displays also have difficulty surviving extremes oftemperature. When the temperature of a display is cycled between coldand hot it will sometimes develop small voids between the spacers andthe liquid crystal fluid. While the voids are small in size, theytypically are noticeable enough that the display will be returned forrepair. The voids are due to the mismatch in the thermal coefficients ofexpansion between the LC and the typically glass or plastic spacers.When a glass substrate display is assembled at room temperature and thensealed, its volume is essentially fixed at that point. As the display iscooled down, both the LC and spacer material will contract but due tothe mismatch in the thermal coefficients of expansion and the mechanicaldiscontinuity at the spacers, stress is localized around the spacers andvoids develop. Initially, the voids are small areas of vacuum or verylow pressure, but the more volatile components of the LC quickly move toa gaseous phase to fill the void to achieve a lower energy equilibriumstate. When the display is returned to room temperature, the vaporfilling the voids prevents the voids from being absorbed back into theLC, and the damage is typically permanent. Display manufactures havesolved this problem by, amongst other methods, utilizing speciallyfabricated spacers that have a softer, more compliant exterior coatingsurrounding a core of either glass or plastic. The outer compliant layeracts to relieve the stresses encountered during thermal cycling of thedisplay, thus preventing the voids. Because of the difficulty ofmanufacturing these spacers, they are often 10-20 times more expensivethan regular spacers and so are often used only when absolutelynecessary.

SUMMARY OF THE INVENTION

In general, in a first aspect, the invention features a liquid crystaldisplay device comprising two substrates facing and spaced from eachother, at least one of the substrates being transparent, electrodespositioned to establish an electric field in the space between the twosubstrates, one or more spacer elements located between the substrates,the spacer elements having been introduced between the substrates duringassembly of the device, an electrooptic material filling at least aportion of the space between the two substrates, and a polymericmaterial filling at least a portion of the space between the substrates,the polymeric material comprising a liquid prepolymeric material thatwas applied to the spacer elements in liquid form and having beenpolymerized in situ after the spacer elements were in place between thesubstrate.

In general, in a second aspect, the invention features manufacturing aliquid crystal display device by introducing spacer elements between twosubstrates that face each other, at least one of the substrates beingtransparent, applying a liquid prepolymeric material to the exteriorsurfaces of one or more spacer elements before or after introduction ofthe spacer elements between the substrates, positioning electrodes toestablish an electric field in the space between the two substrates,filling at least a portion of the space between the two substrates withan electro-optic material, and polymerizing the liquid prepolymericmaterial in situ to form solid polymeric material filling at least aportion of the space between the substrates.

In preferred implementations, one or more of the following features maybe incorporated. The polymeric material may be in the vicinity of thespacer elements. The liquid prepolymeric material may be applied to thespacer elements prior to their introduction between the substrates. Theliquid prepolymeric material may be applied to the spacer elements aftertheir introduction between the substrates. The liquid prepolymericmaterial may be encased in a collapsible shell surrounding at least someof the spacer elements. The prepolymeric material can constitute one ormore of the following materials: monomers (e.g., basic polymer chainmaterial), oligomers (e.g., for cross linking), inhibitors (e.g., toprevent polymerization during storage), adhesion promoters, andpolymerization initiating or enhancing (PIE) materials. The liquidprepolymeric material may have a viscosity equal to or less than2,000,000 centipoise. A polymerization initiating or enhancing (PIE)material may be brought into contact with the liquid prepolymericmaterial. The PIE material may be brought into contact with the liquidprepolymeric material in one of the following ways: it is mixed with theliquid prepolymeric material applied to the spacer elements; it iscarried on or within the spacer elements; it is dissolved or suspendedin the electrooptic material. The liquid prepolymer material and the PIEmaterial may both be encased in a collapsible shell surrounding at leastsome of the spacer elements. The polymerization in-situ may compriseinitiating polymerization by application of light. The liquidprepolymeric material may be a thermoset material, and the polymerizingin situ may comprise the application of heat. The polymeric material maycomprise polymer supports that extend between the two substrates. Thepolymeric material may comprise polymer members that do not extendbetween the two substrates. Additional spacer elements withoutprepolymeric material may be introduced between the substrates. Thespacer elements may comprise a large number of generally spherical orcylindrical elements. The spacer elements may comprise glass. The glassmay be etched. The spacer elements may comprise plastic. The plastic maybe porous. The spacer elements may comprise high-surface area particlesthat are nanoporous, mesoporous, or microporous. The spacer elements maybe randomly located in the space between the substrates. The spacerelements may comprise a large number of elements generally of smallerdiameter than the space between the substrates. The spacer elements maycomprise a large number of elements randomly positioned across the spacebetween the substrates. The spacer elements may generally not be incontact with the substrates. The spacer elements may be in contact withonly one substrate. The spacer elements may comprise a lattice networkstructure. The lattice network structure may be two-dimensional. Thelattice network structure may be three-dimensional. The spacer elementsmay be non-uniform in size and shape. The spacer elements may have arough surface. Most of the spacer elements may be free to move around inthe space between the substrates prior to polymerization. A porousmembrane may serve as a spacer element. The porous membrane may be anextensible porous membrane. The spacer elements may be located innon-image areas of the substrate. The spacer elements may be locatedalong the peripheries of the substrates and serve as one or more sealingmembers sealing the space between the substrates. The spacer elementsmay be located at interpixel regions. The prepolymer may contract uponin situ polymerization. The majority of the polymer supports may bebonded to each of the two substrates. The polymer supports may beprimarily separate members not interconnected with one another. One ormore interconnecting regions of polymer may interconnect a majority ofthe polymer supports. One of the interconnecting regions may comprise alayer of polymer adjacent one of the substrates. The spacer elements maybe dry sprayed on to the substrate before application of theelectrooptic material. The spacer elements may be wet sprayed on to thesubstrate. A solvent may be used for wet spraying comprises theprepolymeric material in solution or suspension. The PIE material maycomprise one or both of the following: an initiator and an accelerant ofthe in situ polymerization process. The PIE material may be lightactivated. The PIE material may comprises a photoinitiator. Thephotoinitiator may comprise a plurality of photoinitiators of differentspectral sensitivities, so that polymerization may be initiated atdifferent times in different locations. The light may be ultravioletlight. The PIE material may be heat activated. The PIE material may beself-activated after a period of time following assembly of the display.The PIE material may comprise both a photoinitiator and an accelerant.The prepolymeric material may applied to the substrates by at least oneof the following processes: pipette, syringe, or printing. The printingmay comprise a silk screen, gravure, flexographic, or lithographicprocess. The spacer elements may be porous structures with a porousmatrix, and the prepolymeric material may absorbed into the porousmatrix of the porous structures. The porous structures may be nanoporousceramic or silica based materials. The spacer elements may comprise anopen network of polymer spheroids formed so that the electroopticmaterial fills interpolymer regions. The porosity of the porousstructure may be selected to yield a desired adhesion of the spacerelement to a polymer matrix comprising the in situ polymerized material.The electrooptic material may be a liquid crystal material. Theelectrooptic material may be a mesomorphic material. The invention mayfurther comprise at least one electrode on at least one substrate togenerate the electric field and at least one electrode on the secondsubstrate. The prepolymeric material may comprises at least one of thefollowing: acrylic-based adhesive, epoxy-based adhesive, urethane-basedadhesive. The prepolymeric material may be primarily cured byapplication by one of the following: light, heat, intermixing of achemical additive. The substrates may comprise a flexible polymermaterial. The display may be capable of withstanding the flexing textreferenced in the detailed description.

Two important specifications that impact a plastic display's durabilityare its compressive and peel strength. In addition, when spacer elementsare a single size and are in contact with both substrates in theassembled cell, it is often the case that the compressive strength isachieved with a lower spacer density than peel strength. It is thereforedesirable to be able to independently improve a display's compressionand peel strength while reducing contrast degradation. Spacer elementscomposed of the same material as the surrounding polymer provide thesignificant benefit of additional polymer interconnections between thesubstrates resulting in added peel strength but with relatively littleimpact on compressive strength. By adjusting the relative densities ofthe spacers and the polymer spacers and the durometer of the polymerspacers, improvements in compressive and peel strengths of the displaydevice's laminate structure can be achieved.

Use of liquid adhesives provides a number of benefits over solidadhesives such as hot melts or thermoplastic adhesives. Liquid adhesivescan achieve significantly better surface adhesion, elongation beforebreak, and tensile strength, for instance, than solid adhesives; theseperformance parameters are all important factors determining how durablethe adhesive bond will be.

Other features and advantages of the invention will be apparent from thefollowing detailed description and from the claims, and from thedisclosure and claims of my applications entitled “ElectroopticalDisplays with Polymer Localized in Vicinities of Substrate Spacers,”U.S. Ser. No. 09/882,310; Electrooptical Displays Constructed withPolymerization Initiating and Enhancing Elements Positioned BetweenSubstrates,” U.S. Ser. No. 09/882,272; and “Electrooptical Displays withMultilayer Structure Achieved by Varying Rates of Polymerization and/orPhase Separation,” U.S. Ser. No. 09/883,083, filed on even date herewith(and incorporated by reference).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a liquid crystal display device thatuses spherical spacers coated with a prepolymeric material afterlamination prior to exposure to the developing light.

FIG. 2 shows a cross section of a liquid crystal display device thatuses spherical spacers coated with a prepolymeric material includingpolymer spacer elements (PSEs) wherein the PSEs are applied prior todepositing the prepolymeric material. The view is shown just prior tolamination.

FIG. 3 shows a cross section of a liquid crystal display device thatuses spherical spacers coated with a prepolymeric material includingpolymer spacer elements (PSEs) wherein the PSEs are applied afterdepositing the prepolymeric material.

FIG. 4 shows a cross section of a liquid crystal display device thatuses spherical spacers mixed into the liquid crystal with polymer spacerelements (PSEs) deposited onto the opposing substrate.

FIG. 5 shows a cross section of a liquid crystal display device thatuses a mesh-like spacing membrane.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, in the preferred embodiment, aliquid crystal display device is assembled using the followingprocedure:

1. The substrates are a flexible polymer material with a low level ofbirefringence to improve the optical qualities of the final product andhaving a glass transition temperature greater than 150 degrees C inorder to facilitate the various drying and baking operations. A polymerthat meets these requirements is poly ether sulphone (PES). A vaporbarrier is coated onto the outside surface of the substrate to improvethe reliability and product life of the display; the vapor barrier istypically composed of a thin film laminate structure of silicon oxideand another polymer.

2. The substrates are coated with a vacuum-deposited layer of typicallyindium tin oxide (ITO), which is a transparent conductor. The ITO isthen patterned via chemical, electron beam, or laser etching.

3. The prepolymer may be a commercially available photocured polymersuch as Norland Product's NOA-65 or it may be a custom formulation suchas the following:

% Material 50 Sartomer Co. SR 9644 15 Sartomer Co. SR 306 18.5 SartomerCo. SR 256 1.25 Stabilizer 3 Aceto Quantacure ITX 5.5 Tri ethanol amine(TEA) 1.05 3M Corp. FC-430 1.6 Dow Corning DC-57 1.6 Wetting Agent 2.5Fratelli Lamberti Escacure KIP

4. The prepolymer mixture is diluted in a solvent such as methanol at aratio sufficient to achieve a viscosity of approximately 50 cps.

5. A polyimide solution is coated onto the ITO side of at least one ofthe substrates and baked at a temperature of 150 degrees C for one hour.The polyimide surface(s) are then rubbed to develop an alignment layerfor the liquid crystal.

6. A liquid crystal such as Merck E7 is coated in a thin layer onto thepolyimide surface. The exact quantity of liquid crystal is not criticalsince the thickness of the cell is determined by the spacer elements.

5. Glass spacers of a diameter of 3-3.5 μm are surface-etched using a1.25% solution of hydrofluoric acid for 10 minutes while suspended insolution in an ultrasonic vibration tank. After washing, the etchedspacers are then coated with a mixture of methacrylate silane and aphotoinitiator by immersing the spacers into a solution containing theinitiator and an adhesion promoter such as a silane and then sprayingthe spacers into the top of a vertical drying column onto thesubstrate(s). Silanes improve the bonding between glass and polymers viachemical bonding at the silane/glass interface and a dispersion of thepolymer into the silane at the silane/polymer interface.

6. Spacers are mixed into the prepolymer solution in a concentration ofapproximately 1:1 wt/wt. The spacers are then dispersed into theprepolymer solution using ultrasonic mixing.

7. Using an aerosol spray system, the prepolymer/spacer mixture is thensprayed onto the uncoated substrate for a sufficient time to achieve asurface density of at least about 30 spacers/mm2 when the display hasbeen assembled. The spacers tend to be distributed generally randomlyacross the substrate surface. Micro-filtered compressed nitrogen atapproximately 10-30 psi is used as the propellant. The pressure,viscosity of the mixture, relative concentrations of spacers andprepolymer, as well as nozzle orifice shape are adjusted to achieve theprepolymer coating single spacers with some number of dropletscontaining prepolymer only. That substrate is then exposed to 120degrees centigrade for 15 minutes to remove the solvents from themixture.

8. The substrate with the prepolymer/spacer mix will typically undergo apre-cure step to provide a tack to the polymer prior to the laminationstep. If the prepolymer is curable by both heat and light, then thesolvent bake step can be used as the pre-cure step as well.

9. The substrates are then laminated together while maintaining theproper alignment between the ITO patterns on the upper and lowersubstrates.

8. Both sides of the cell are then exposed to UV light that causesscission of the photoinitiator and release of free radicals around thespacers. The polymerization reaction will then proceed with theinitiation sites centered around each spacer that was deposited with theinitiator. FIG. 1 depicts a cross-sectional view of the display afterstep 8.

The resulting display is quite flexible. It can be flexed withoutpermanent damage by at least the amount of flexing specified in theflexing tests described in U.S. Pat. No. 6,019,284, hereby incorporatedby reference.

It is not necessary that polymer form in the vicinity of each spacer,nor that the polymer extend fully from one substrate to another in allcases. Some spacers, for example, may not have been coated withprepolymeric material, or they may have been imperfectly coated.

Polymer supports that do not extend fully from one substrate to theother may still be of benefit in creating isolated regions of liquidcrystal, and thereby make possible improved bistability of certainferroelectric liquid crystal materials, which may exhibit improvedbistability if the liquid crystal layer is divided into discretedroplets along one substrate.

In another embodiment, polymer spacer elements (PSEs) smaller than thepredominant spacer element size that determines the substrate spacingcan be added to the prepolymer/spacer mix. These PSEs are composed of apolymer formulated to typically achieve a transparency and an index ofrefraction that is the same as the polymer and liquid crystal to reducedispersion. These polymer spacing elements can be produced by a numberof methods and may even be formed at the time of their deposition ontothe substrate via aerosol dispersion followed by heat the substrate, theprepolymer/spacer mix is deposited as before. Alternatively, the PSEsmay be and ultraviolet cure of the polymer microspheres generated. Oncethe PSEs are deposited onto mixed with the prepolymer/spacer mix priorto its deposition onto the substrate. This embodiment allows foradditional points of polymer contact between the substrates without anyadditional rigid spacer elements that can reduce the contrast of thedisplay. This can be seen in FIG. 2.

In a separate process step, additional PSEs may be deposited. In thiscase, the aerosol conditions are adjusted so that the droplet sizeresults in the tops of PSEs being slightly higher than the desiredspacing between the substrates. Additionally, the PSEs are depositedonto the surface in a semi-cured state. Upon lamination, the prepolymercovered spacers are pushed through the liquid crystal layer and pressedagainst the opposing substrate as in the previously describedembodiments; the semi-cured PSEs are also pushed through the liquidcrystal layer and compressed by the opposing substrate. The PSEs may becomposed of a polymer that, while having similar optical properties asthe polymer coating the spacers, may have different mechanicalproperties, for instance, the PSE material may be formulated to shrinkupon curing as well as having a value for its elongation before breakparameter. In this way, the plastic substrates can actually be drawntogether by a controlled amount subsequent to a full cure of thelaminated cell. This will significantly enhance the flexure performanceof the display as well as its overall durability.

In another embodiment, the spacers either individually or in smallgroups are encased in a collapsible shell that also contains a quantityof prepolymer. This can be achieved by a variety of well-knowntechniques classified generally as microencapsulation.Microencapsulation processes fall into several general categories, allof which can be applied to the present invention: interfacialpolymerization, in situ polymerization, physical processes such ascoextrusion, and coacervation. When the substrates are laminatedtogether with the liquid crystal material and the polymer-coated spacerelements taking the form of the just-described encapsulated elements,the microcapsules are crushed during the lamination process releasingthe prepolymer. As in other embodiments the spacers are pressed againstboth substrates during lamination, while the released prepolymer remainssurrounding the spacer elements and is being brought into contact withthe two substrates. The display is then exposed to ultraviolet light asbefore and the prepolymer is polymerized, resulting in the cured polymerbonding the spacer to both substrates.

While other microencapsulation techniques can be used to createmicrocapsules with the appropriate characteristics for this invention,two encapsulation techniques that are particularly well suited to thepresent invention are in-situ polymerization and interfacialpolymerization.

The technique of in situ polymerization utilizes an oil/water emulsion,which is formed by dispersing the spacer/prepolymer mix in an aqueousenvironment. Prior to emulsifying the prepolymer in the aqueous phase,the spacers are mixed and fully dispersed into the prepolymer in a ratioof 1:2 wt/wt. The resulting prepolymer droplets due to emulsificationwill be 15-30 microns in diameter with less than 5 spacers containedwithin the droplet. A goal of the process is to create capsules withonly one spacer per capsule. Tighter control of process parameters canachieve less deviation on droplet size which will minimize capsules withmore than one spacer. By using a spacer composed of a material ofdifferent density than the prepolymer, e.g. glass, the number ofcapsules not containing one spacer can be minimized by sorting thefinished capsules via such density-sorting methods as flotation orcentrifuge. Monomers are introduced into the aqueous phase, polymerize,and form a polymer with higher affinity for the internal phase than forthe aqueous phase, thus condensing around the emulsified oily droplets.In one especially useful in situ polymerization processes, urea andformaldehyde condense in the presence of poly (acrylic acid) (See, e.g.,U.S. Pat. No. 4,001,140). The resulting capsule wall is aurea/formaldehyde copolymer, which encloses the spacer/prepolymerdroplet. The capsule is clear and rigid. Transparency is important sincethe capsule shell will remain in the display after lamination; rigidityis an important feature of the capsule shell since an overly compliantshell material will result in the shell not crushing and properlyreleasing the prepolymer during lamination. Melamine-based shellmaterials also have the required qualities transparency and rigidity.

The electro-optic material used in the display can be any type of liquidcrystal; for instance, the invention provides benefits to nematic,twisted nematic, super-twisted nematic, ferrolectric,anti-ferroelectric, cholesteric liquid crystal displays, to name a few.The invention also provides benefits to non-LCD displays. For instance,the durability of electrophoretic and Gyricon displays can besignificantly enhanced by this method.

In another preferred embodiment, a liquid crystal display device isassembled using the following procedure:

1. The substrates are a flexible polymer material with a low level ofbirefringence to improve the optical qualities of the final product andhaving a glass transition temperature greater than 150 degrees C inorder to facilitate the various drying and baking operations. A polymerthat meets these requirements is poly ether sulphone (PES). A vaporbarrier is coated onto the outside surface of the substrate to improvethe reliability and product life of the display; the vapor barrier istypically composed of a thin film laminate structure of silicon oxideand another polymer.

2. The substrates are coated with a vacuum-deposited layer of typicallyindium tin oxide (ITO), which is a transparent conductor. The ITO isthen patterned via chemical, electron beam, or laser etching.

3. The prepolymer may be a commercially available photocured polymersuch as Norland Product's NOA-65 or it may be a custom formulation suchas the following:

% Material 50 Sartomer Co. SR 9644 15 Sartomer Co. SR 306 18.5 SartomerCo. SR 256 1.25 Stabilizer 3 Aceto Quantacure ITX 5.5 Tri ethanol amine(TEA) 1.05 3M Corp. FC-430 1.6 Dow Corning DC-57 1.6 Wetting Agent 2.5Fratelli Lamberti Escacure KIP

4. The prepolymer mixture is diluted in a solvent such as methanol at aratio of 1:25 parts by weight.

5. A polyimide solution is coated onto the ITO side of at least one ofthe substrates and baked at a temperature of 150 degrees C for one hour.The polyimide surface(s) are then rubbed to develop an alignment layerfor the liquid crystal.

6. Glass spacers of a diameter of 3-3.5 μm are used. The spacers aremixed into a liquid crystal such as Merck E7 in large numbers (with adensity sufficient to produce a surface density of at least about 30spacers/mm2 when the display has been assembled). The spacers tend to bedistributed generally randomly across the substrate surface.

7. The liquid crystal/spacer mix is coated in a thin layer onto thepolyimide surface. The exact quantity of liquid crystal is not criticalsince the thickness of the cell is determined by the spacer elements.

8. The PSEs are deposited onto the uncoated substrate via aerosoldispersion. The viscosity of the prepolymer formulation is adjusted andthe prepolymer is diluted, as necessary, in a solvent in addition toadjusting the aerosol parameters to achieve particle sizes of the PSEsthat are approximately 30% larger than the spacer diameter. Thatsubstrate is then exposed to 120 degrees centigrade for 15 minutes toremove the solvents from the mixture.

9. The substrate with the PSEs will typically undergo a pre-cure step toprovide a tack to the polymer prior to the lamination step. If theprepolymer is curable by both heat and light, then the solvent bake stepcan be used as the pre-cure step as well.

10. The substrates are then laminated together while maintaining theproper alignment between the ITO patterns on the upper and lowersubstrates. FIG. 4 depicts the substrates just prior to lamination.

11. Both sides of the cell are then exposed to UV light that causesscission of the photoinitiator and release of free radicals around thespacers. The polymerization reaction will then proceed with theinitiation sites centered around each PSE.

Two or more photoinitiators with different spectral sensitivities may beused to control when polymerization is initiated at a particular site.Since scission of the photoinitiator occurs when the photon energy ofthe light source exceeds a certain threshold, photoinitiators willtypically be sensitive to light of wavelengths less than a specificvalue; thus, a photoinitiator sensitive to visible light will usuallyalso be sensitive to ultraviolet light. One embodiment using thisfeature would be to coat the spacer elements with a UV sensitivephotoinitiator and to have the prepolymer of the PSEs contain a visiblelight sensitive photoinitiator. The assembly is first exposed to visiblelight, resulting in the curing of only the PSEs, which, due to theshrinkage of the PSE polymer, draw the two substrates together. Theassembly is then exposed to UV light, causing the polymer surroundingthe spacers to be cured.

In another embodiment, other polymerization enhancing compounds such asadhesion promoters, or additives such as urethanes which improveelongation before tear properties are added to all, or some subset, ofthe PSEs. In such a way, peel strength can be further enhanced.

One possible polymer are acrylic adhesives which have excellent opticalclarity as well as the availability of a wide selection of manufacturedoptical grade versions of the material. Other polymers that might alsobe used are, for instance, epoxies or urethanes, though typically theseclasses of polymers do not have the optical properties equal to those ofthe acrylics. Acrylic adhesives are reactive cross-linking structuraladhesives that cure by means of free-radical initiation. They are basedon the methacrylate monomers and cure by addition polymerization. Theformation of free radicals initiates a sudden and rapid chain reactionand curing of the adhesive. Condensation polymerization, on the otherhand, typified by urethane and epoxies, proceeds at an approximatelyconstant, usually lower reaction rate. Generation of free radicals forinitiation of polymerization of acrylic based adhesives can beaccomplished by a redox reaction such as that involving dimethyl anilineand peroxide. Because of the nature of the chain reaction, the freeradicals can propagate from monomer to monomer and the cure itself canpropagate up to 2.5 mm from the point of polymerization initiation. As aresult of this cure propagation phenomenon, the accelerator and monomerdo not have to be fully intermixed to achieve a full cure. This leads toseveral other methods for curing, where the accelerator can be in theform of a lacquer or thin layer on one surface allowing for the primingand storing of parts. In another related cure method termed ‘honeymoon’or ‘no-mix’ in industry parlance, a two part adhesive is used which whenbrought into contact with each other (without any intermixing necessary)will result in the generation of sufficient free radicals to fullypolymerize all the material.

Acrylics can also be cured by exposure to ultraviolet light less than400 nm in wavelength, and in some instances by light in the visiblerange as well. In the case of photocurable adhesives, the free radicalsource is termed a photoinitiator and results in the formation of freeradicals on exposure to light. Compounds which act as photoinitiatorswith light in the range of 200-300 nm are benzoine ethers, activatedbenzophenones and related compounds; benzyl dialkyl amino morpholinylketone is an example of a visible wavelength-activated photoinitiator.Photoinitiators are disassociated into segments forming free radicals bylight in a process known as scission. One example of an equal mix curingsystem is embodied in U.S. Pat. No. 4,331,795 which uses a cobalt saltaccelerator in one component and a hydroperoxide in the other element.Epoxies may also be formulated that can be UV-cured via cationicpolymerization by incorporating reactive diluents and cyclic monomers.UV-initiated cationic curing of urethanes may be accomplished, forinstance, by basing the formation on vinyl ether and polyurethaneoligomers such as that manufactured by Allied Signal Inc.

A great variety of embodiments of the invention may be practiced. Therate of photopolymerization may be controlled by adjusting the intensityof the light source. The spacer elements may be porous structures, andthe prepolymer is then allowed to absorb into the porous matrix in orderto provide better interpenetration of the polymer and spacing, thusproviding better adhesion. The spacer elements may be composed of glass,typically in the form of beads or rods, which are then etched toincrease the surface area for improved adhesion. One or more layers of aan adhesion promoter such as a silane coupling agent may be coated ontothe glass spacers which may or may not have been etched, prior to thecoating of the glass spacers with the prepolymer. The spacer elementsmay be admixed to the prepolymer in concentrations higher than whatwould be desired in regions of the display that are active image areas;the mixture is then deposited onto the substrate via printing or pipettemethods into the interpixel regions or the perimeter where no image ispresented, thus provided additional support without adversely affectingthe image contrast or quality. The initiator may be solely heatactivated or heat activated as well as photo-activated or otheractivation method. The polymer is chosen so as to contract followinginitial bonding to the substrates and upon curing; the two substratesare thus drawn together, increasing durability of the display; this isparticularly effective when the polymer is localized around the spacerelement, as has been previously described. The spacer element may be oneor more sheets of an extensible porous membrane that when laminated inbetween the substrates is the element that determines the spacingbetween the substrates. One or more of the substrates may be of glass orother rigid material.

Other embodiments of the invention are within the following claims.

What is claimed is:
 1. A method of manufacturing a display devicecontaining a material with optical properties that are influenced by thefield, comprising: positioning electrodes to establish the field in thespace between two substrates that face each other, at least one of thesubstrates being transparent; introducing spacer elements between thetwo substrates, the spacer elements being generally randomly distributedbetween the substrates; applying a liquid comprising a prepolymericmaterial to the exterior surfaces of the spacer elements beforeintroduction of the spacer elements between the substrates, the liquidbeing in a liquid state upon introduction of the spacers into the spacebetween the substrates; prior to introducing the material with opticalproperties that are influenced by the field, polymerizing theprepolymexic material in situ to form solid polymeric material occupyingat least a portion of the space between the substrates and providing abond between the substrates, at least a portion of the solid polymericmaterial being in the vicinities of the spacer elements; and thenfilling at least a portion of the space between the two substrates withthe material with optical properties that are influenced by the field.2. The subject matter of claim 1 wherein the liquid comprising theprepolymeric material is encased in a collapsible shell surrounding atleast some of the spacer elements.
 3. The subject matter of claim 1wherein a polymerization initiating or enhancing (PIE) material isbrought into contact with the liquid prepolymeric material.
 4. Thesubject matter of claim 3 wherein the PIE material is brought intocontact with the prepolymeric material in one of the following ways: itis mixed with the liquid comprising the prepolymeric material applied tothe spacer elements; it is carried on or within the spacer elements. 5.The subject matter of claim 4 wherein the liquid comprising theprepolymer material and the PIE material are both encased in acollapsible shell surrounding at least some of the spacer elements. 6.The subject matter of claim 3 wherein the PIE material comprises one orboth of the following: an initiator and an accelerant of the in situpolymerization process.
 7. The subject matter of claim 6 wherein the PIEmaterial is light activated.
 8. The subject matter of claim 7 whereinthe PIE material comprises a photoinitiator.
 9. The subject matter ofclaim 8 wherein the photoinitiator comprises a plurality ofphotoinitiators of different spectral sensitivities, so thatpolymerization may be initiated at different times in differentlocations.
 10. The subject matter of claim 7 wherein the light isultraviolet light.
 11. The subject matter of claim 6 wherein the PIEmaterial is heat activated.
 12. The subject matter of claim 6 whereinthe PIE material is self activated after a period of time followingassembly of the display.
 13. The subject matter of claim 3 wherein thepolymerization in-situ comprises initiating polymerization byapplication of light.
 14. The subject matter of claim 1 wherein theliquid prepolymeric material is a thermoset material, and thepolymerizing in situ comprises the application of heat.
 15. The subjectmatter of claim 1 wherein the polymeric material comprises polymersupports that extend between the two substrates.
 16. The subject matterof claim 15 wherein the majority of the polymer supports are bonded toeach of the two substrates.
 17. The subject matter of claim 15 whereinthe polymer supports are primarily separate members not interconnectedwith one another.
 18. The subject matter of claim 15 wherein one or moreinterconnecting regions of polymer interconnects a majority of thepolymer supports.
 19. The subject matter of claim 18 wherein one of theinterconnecting regions comprises a layer of polymer adjacent one of thesubstrates.
 20. The subject matter of claim 1 wherein to polymericmaterial comprises polymer members that do not extend between the twosubstrates.
 21. The subject matter of claim 1 wherein additional spacerelements without prepolymeric material are introduced between thesubstrates.
 22. The subject matter of claim 1 wherein the spacerelements comprise a large number of generally spherical or cylindricalelements.
 23. The subject matter of claim 22 wherein the spacer elementscomprise glass.
 24. The subject matter of claim 23 wherein the glass isetched.
 25. The subject matter of claim 22 wherein the spacer elementscomprise plastic.
 26. The subject matter of claim 25 wherein the plasticis porous.
 27. The subject matter of claim 26 wherein the spacerelements comprise high-surface area particles that are nanoporous,mesoporous, or microporous.
 28. The subject matter of claim 25 whereinthe spacer elements comprise PSEs.
 29. The subject matter of claim 22wherein the spacer elements are randomly located in the space betweenthe substrates.
 30. The subject matter of claim 1 wherein the spacerelements comprise a large number of elements generally of smallerdiameter than the space between the substrates.
 31. The subject matterof claim 30 wherein the spacer elements comprise a large number ofelements randomly positioned across the space between the substrates.32. The subject matter of claim 30 wherein the spacer elements aregenerally not in contact with the substrates.
 33. The subject matter ofclaim 30 wherein the spacer elements are in contact with only onesubstrate.
 34. The subject matter of claim 1 wherein the spacer elementscomprise a lattice network structure.
 35. The subject matter of claim 34wherein the lattice network structure is two-dimensional.
 36. Thesubject matter of claim 34 wherein the lattice network structure isthree-dimensional.
 37. The subject matter of claim 1 wherein the spacerelements are non-uniform in size and shape.
 38. The subject matter ofclaim 1 wherein the spacer elements have a rough surface.
 39. Thesubject matter of claim 1 wherein most of the spacer elements are freeto move around in the space between the substrates prior topolymerization.
 40. The subject mater of claim 1 wherein a porousmembrane serves as a spacer element.
 41. The subject matter of claim 40wherein the porous membrane is an extensible porous membrane.
 42. Thesubject matter of claim 1 wherein the spacer elements are located innon-image areas of the substrate.
 43. The subject matter of claim 42wherein the spacer elements are located along the peripheries of thesubstrates and serve as one or more sealing members sealing the spacebetween the substrates.
 44. The subject matter of claim 42 wherein thespacer elements are located at interpixel regions.
 45. The subjectmatter of claim 1 wherein the prepolymer contracts upon in situpolymerization.
 46. The subject matter of claim 1 wherein the spacerelements are wet sprayed on to the substrate.
 47. The subject matter ofclaim 6 wherein a solvent used for wet spraying comprises theprepolymeric material in solution or suspension.
 48. The subject matterof claim 1 wherein the PIE material comprises both a photoinitiator andan accelerant.
 49. The subject matter of claim 1 wherein the spacerelements are porous structures wit a porous matrix, and the prepolymericmaterial is absorbed into the porous matrix of the porous structures.50. The subject matter of claim 49 wherein the porous structures arenanoporous ceramic or silica based materials.
 51. The subject matter ofclaim 1 wherein the spacer elements comprise an open network of polymerspheroids formed so that the material with optical properties that areinfluenced by the field fills inter-polymer regions.
 52. The subjectmatter of claim 1 wherein the electro optic material is a liquid crystalmaterial.
 53. The subject matter of claim 1 wherein the electroopticmaterial is a mesomorphic material.
 54. The subject matter of claim 1wherein the prepolymeric material comprises at least one of thefollowing: acrylic-based adhesive, epoxy-based adhesive, urethane-basedadhesive.
 55. The subject matter of claim 1 wherein the prepolymericmaterial primarily cured by application by one of the following: lightheat, intermixing of a chemical additive.
 56. The subject matter ofclaim 1 wherein the substrates comprise a flexible polymer material. 57.The subject matter of claim 56 wherein the display is capable ofwithstanding the flexing text referenced in the detailed description.58. The subject matter of claim 1 wherein the prepolymeric material is aliquid when applied to the spacer elements.
 59. The subject matter ofclaim 58 wherein the liquid prepolymeric material comprises one or moreof the following: monomer, oligomer, inhibitor, adhesion promoter,polymerization initiating or enhancing (PIE) material.
 60. The subjectmatter of claim 58 wherein the liquid prepolymeric material has aviscosity equal to or less than 2,000,000 centipoise.
 61. The subjectmatter of claim 49 wherein the porosity of the porous structure isselected to yield a desired adhesion of the spacer element to a polymermatrix comprising the in situ polymerized material.
 62. The subjectmatter of claim 1 further comprising at least one electrode on thesecond substrate.
 63. The subject matter of claim 1 wherein the liquidcomprising the prepolymeric material has a viscosity and compositionsuited for adhering to the surfaces of the spacer elements as the spacerelements are spray deposited on a substrate.
 64. The subject matter ofclaim 1 wherein the spacer elements to which the liquid comprisingprepolymeric material is applied are made from a material other thanpolymer and there are additional polymer spacer elements.
 65. Thesubject matter of claim 58 wherein the liquid prepolymeric material isdissolved in a solvent when applied to the spacer elements.
 66. Thesubject matter of claim 64 wherein the liquid comprising prepolymericmaterial is also applied to the polymer spacer elements.
 67. The subjectmatter of claim 1 wherein the spacer elements comprise a core with atleast one coating, and the liquid comprising the prepolymeric materialis applied over the coating.
 68. The subject matter of claim 67 whereinthe coating comprises a material selected to improve the bond betweenthe spacers and the liquid comprising the prepolymeric material orbetween the spacers and polymeric material.
 69. The subject matter ofclaim 1 wherein the material with optical properties that are influencedby the field is an electrooptic material.
 70. The subject matter ofclaim 1 wherein the material with optical properties that are influencedby the field is an electrooptic material.
 71. The subject matter ofclaim 1 wherein the spacer elements comprise a large number of elementsgenerally of the same or smaller diameter than the space between thesubstrates.
 72. The subject matter of claim 71 wherein the spacerelements are coated individually with the liquid comprising theprepolymeric material prior to introduction of the spacer elementsbetween the substrates.
 73. The subject matter of claim 72 wherein thespacer elements are wet sprayed with the liquid coating on to at leastone of the substrates.
 74. The subject matter of claim 73 wherein theelectrodes are positioned on both substrates.
 75. The subject matter ofclaim 1 wherein the spacer elements comprise a large number of elementsgenerally of the same or smaller diameter than the space between thesubstrates, and the spacer elements are wet sprayed on to one of thesubstrates along with the liquid comprising the prepolymeric material.76. The subject matter of claim 1 wherein the electrodes are configuredto establish an electric field in the space between the substrates. 77.The subject matter of claim 1 wherein the electrodes are configured toestablish a magnetic field in the space between the substrates.
 78. Thesubject matter of claim 1 wherein the liquid comprising the prepolymericmaterial is introduced into the space between the substrates in a liquidstate at a temperature below 120 degrees C.